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It seems Cadence looks at the entire cline (vs.cline segments), when applying spacing constraints within a region. This isn't good, but it's what you have to work with.

Off hand, a quick work around would be to terminate the cline, where the cline width changes inside the constraint region. For example, route the trace at the width desired inside the constraint region. Before the cline trace exits this region, place a "via" pad that will serve to terminate the cline. From this via, change to the alternate line width and continue routing outside the constriant region.

To make the via, define it on a single routing layer with no drill. Make the pad size equal to, or less than, the smaller line width. Simply select this via while routing, to terminate the clines as needed. Update the shapes and you should have the proper line-to-shape clearance both inside and outside the contraint region.

Last step is to ask Cadence to revise the software, so that constraint regions consider cline segments vs. clines.